(1) Field of the Invention
The present invention relates to a melamine resin foam, a process for production thereof, and a melamine/formaldehyde resin suitably used in the production of said melamine resin foam.
(2) Description of the Prior Art
Melamine resin foams are produced from a resin composition comprising, as main components, a melamine/formaldehyde resin (which is a thermosetting resin obtained by the reaction of melamine and formaldehyde) and a blowing agent. Among organic resin foams, melamine resin foams show excellent flame retardancy and are regarded to be useful particularly as a heat insulating construction material. The melamine resin foams, however, are very fragile generally with the fragility being conspicuous in the case of highly expanded foams, causing cracking. Hence, the melamine resin foams see no practical application yet in spite of their excellent flame retardancy mentioned above.
For the realization of the practical application of melamine resin foams, many researches have been made. For example, in Japanese Patent Application Kokai (Laid Open) No. 17697/1978 is disclosed a process for producing a melamine resin foam by adding, to a melamine/formaldehyde condensate, 12%-330% by weight, based on the resin, of an isocyanate and giving rise to foaming in the presence of a catalyst using no blowing agent. However, addition of such a large amount of an isocyanate produces a melamine resin foam of low flame retardancy.
Further in Japanese Patent Publication No. 50943/1990 is disclosed a process for producing an elastic foam (which uses a melamine/formaldehyde condensate as a basic material and which is free from problems of fragility and flame retardancy and has good properties) by applying an ultrahigh frequency to a solution comprising a melamine/formaldehyde precondensate, a blowing agent and other substances.
As well known, when an ultrahigh frequency is applied to a metal, discharge takes place. The above solution comprising a melamine/formaldehyde precondensate, a blowing agent and other substances contains, in most cases, a combustible solvent (in the above Japanese Patent Publication No. 50943/1990, there are mentioned, as the blowing agent used, hydrocarbons, alcohols, etc.). In this connection, if the above solution containing a melamine/formaldehyde precondensate, a blowing agent and other substances contains a metal, the application of an ultrahigh frequency to the solution causes discharge and quite possibly allows the solution to catch a fire. Thus, the process disclosed in Japanese Patent Publication No. 50943/1990 is not suitable at least for use on industrial scale.